|Or is all the data still going through one pipeline out the house to the box outside? |
Yes, it is. It is the bandwidth between you and your ISP that is the deciding factor.
If you have two different accounts on the same connection you will still be sharing bandwidth. In fact I am not even sure you can have two accounts on the same connection. You would have to speak to your ISP.
One thing definitely cannot do is have two Modems on the same connection. That is not possible with either cable or DSL. The Modems would be fighting each other to synchronise with the ISP Modem at the other end.
The only guaranteed solution is two separate connections coming into the house and all that entails.